The present invention broadly relates to an infrared intrusion detector and more specifically pertains to a new and improved construction of an infrared intrusion detector having an infrared sensor and a plurality of reflectors to focus the infrared radiation from a plurality of radiation receiving zones onto the infrared sensor.
In its more particular aspects, the infrared intrusion detector of the present invention focuses the infrared radiation from a plurality of radiation reception zones through a folded radiation path onto the infrared sensor using discrete primary reflectors and a common secondary reflector. The primary reflectors are aligned in a horizontal row and correspond in number to the number of respective radiation reception zones.
In other words, the infrared intrusion detector of the present invention has a predetermined number of radiation reception zones and comprises an infrared sensor and a plurality of reflectors for focusing infrared radiation arriving from the predetermined number of radiation reception zones onto the infrared sensor. The plurality of reflectors comprises a predetermined number of primary reflectors and the predetermined number of primary reflectors forms at least one substantially horizontal row. The plurality of reflectors also comprises at least one common secondary reflector. The predetermined number of primary reflectors and the at least one common secondary reflector cooperate to form a folded radiation path for focusing the infrared radiation arriving from the predetermined number of radiation reception zones onto the infrared sensor. The predetermined number of primary reflectors corresponds in number to the predetermined number of radiation reception zones.
An infrared intrusion detector of this type is known, for example, from FIG. 5 of the European Pat. No. 0,025,188. By folding the path of radiation, it is possible to reduce the construction depth of the infrared intrusion detector by about one-half in the direction of radiation compared with designs not using a folded radiation path. A relatively flat design of the infrared intrusion detector is thereby achieved. The disadvantage of the disclosed arrangement of a plurality of primary reflectors in a horizontal row is that it is only possible to set up a group of horizontally adjacent radiation reception zones. Using this arrangement it is only possible to incompletely cover an area which is to be monitored with radiation reception zones. It is therefore still possible for an intruder to move about in a monitored area without penetrating into a radiation reception zone and thus to remain unobserved. The protection provided by using such an infrared intrusion detector is therefore incomplete and needs improvement.
An infrared intrusion detector has been disclosed in the U.S. Pat. No. 3,703,718, in which a plurality of horizontal rows of reflectors are provided and which are especially shown in FIGS. 9 to 11. This plurality of horizontal rows of reflectors forms a plurality of groups of radiation reception zones with different angles of inclination from the horizontal. However, because no folded path of radiation is intended or provided, the infrared sensors must be separated from the reflectors by a distance equal to the focal length of the reflectors. In an infrared intrusion detector of medium range, the greatest focal lengths encountered are, as a rule, between six and ten centimeters. The depth of the unit and the dimensions of the infrared intrusion detector can therefore be considerable and render inconspicuous mounting impossible. Furthermore, it is a disadvantage that the focal lengths of the different rows of reflectors are selected nearly equal in order to be able to arrange all reflectors in the same approximate location and on the same support bracket. Since the radiation reception zones at different angles of inclination to the horizontal detect an intruder at different distances from the infrared intrusion detector, the fact that the reflectors of the different horizontal rows have the same focal length results in an intruder, into the corresponding radiation reception zones, being imaged by the reflectors in different sizes onto the infrared sensor. There is no provision for matching the infrared sensor area and the body area of an intruder at different distances from the infrared intrusion detector. Thus, the sensitivity of the infrared intrusion detector is different in radiation reception zones having different angles of inclination.
This disadvantage can be avoided according to U.S. Pat. No. 4,339,748, granted July 13, 1982, by choosing different focal lengths of the reflectors of different horizontal rows. In order to achieve a clear image of an intruder at the common infrared sensor, reflector rows having different angles of inclination for different radiation reception zones must be spaced at correspondingly different distances from the common infrared sensor. This requires a complicated optical design because the individual rows of reflectors have to be located at different locations.
The aforesaid infrared intrusion detectors referred to also have a further disadvantage in that the surface areas of the reflectors of a horizontal row are nearly equal or are not significantly different, or even that the surface areas of the lateral reflectors are considerably smaller than the surface areas of the middle reflectors, as shown in FIG. 9 of the aforementioned U.S. Pat. No. 3,703,718. A lesser amount of infrared radiation is conveyed to the surface of the infrared sensor from the therewith associated radiation reception zones which are inclined or tilted relative to the vertical plane of symmetry of the infrared intrusion detector than from the middle or intermediate radiation reception zones. This is due, on the one hand, to the smaller area of the reflector, and, on the other hand, to the smaller apparent area of the infrared sensor element in relation to radiation impinging obliquely upon the infrared sensor element. The sensitivity of prior art intrusion detectors is therefore significantly less in the lateral radiation receiving zones than in the intermediate or middle radiation receiving zones.